Copper is frequently recovered from extremely low grade ores and from mine wastes by dump leaching. Dump leaching involves percolating a dilute sulfuric acid solution through a heap of cupriferous material and collecting the copper-bearing acid solution after it has percolated through the heap. The addition of ferric ion (generally added as ferric sulfate) to the acid leach solution is common when the material being leached includes sulfide copper minerals. The copper-bearing leach solution after passage through the heap generally is treated with scrap iron to recover the copper by cementation. The copper-depleted acid solution from the cementation operation (after treatment of replenish its acid and ferric ion concentration when such is necessary) often is recycled for further leaching of the heap. U.S. Pat. No. 3,441,316 to Hannifan et al. and U.S. Pat. No. 3,669,651 to Spedden et al. described more or less conventional dump leaching operations.
Similar leaching procedures have been employed also for the recovery of copper from low grade copper-bearing materials by in situ leaching, as described for example in the Scott U.S. Pat. No. 2,563,623 and the Ortloff et al. U.S. Pat. No. 3,574,599.
It is characteristic of dump leaching operations that the total amount of copper recovered generally has not exceeded about half of the copper contained in the dump, and recovery of even this amount usually requires essentially continuous leaching for long periods of time, measured in years. For example, a typical dump leach operation might recover, over a period of 5 years, no more than 40% of the copper content of a dump, and the amount of additional copper recovered by extending the leaching operation for a longer period might not pay for the additional acid used. Such low and slow copper recovery has caused dump leaching to be considered a technique suitable only for mine wastes or other cupriferous material too low grade to merit treatment by conventional concentration and smelting procedures or by other procedures available for treating relatively high grade ores and concentrates.
Recently it has been found that substantially increased recoveries of copper from oxidized ores, containing significant amounts of substantially acid insoluble oxidized minerals (e.g. tenorite and melaconite) can be obtained by aging such oxidized ores in a dump leach heap in contact with a strong acid solution, containing upwards of 100 g/l of sulfuric acid, for a period of several days, and thereafter washing the aged ore with a dilute sulfuric acid solution containing about 5 to 40 g/l H.sub.2 SO.sub.4. Substantially increased recoveries of insoluble or very difficultly soluble oxidized copper minerals are obtained by this treatment in a substantially shorter period of time than by conventional dump leaching operations using only a dilute acid solution as the leaching medium.